As a therapeutic approach for improving symptoms by removing from disease agents and toxic waste materials accumulated in blood due to various causes, an extracorporeal-circulation blood purification therapy has been widely applied.
In the extracorporeal-circulation blood purification therapy, a blood processing apparatus is used. Examples of the blood processing apparatus include a hemodialyzer, a hemofilter, a blood fractionator and a plasma separator. The separation membrane for blood processing is a separation membrane to be charged in a blood processing apparatus. At present, majority of blood processing apparatuses is occupied by a hollow fiber membrane-based blood processing apparatus.
As a membrane base material for the separation membrane for blood processing, a base material mainly comprising a polymer, such as a cellulose polymer, a cellulose acetate polymer, a polyamide polymer, a polyolefin polymer, a polyacrylonitrile polymer and a polysulfone polymer, has been used. Among them, the polysulfone polymer has been increasingly used as a base material for a separation membrane for blood processing in recent years, since it is excellent in membrane-formability in addition to biological safety and chemical stability; and a wide range of membranes can be designed to have a variety of permeability and membrane structures. These polymers are characterized in that they have excellent resistances against radiation, heating and chemical agents such as acids or alkalis; however, since they are hydrophobic polymers, these polymers lack affinity for blood if they are used as they are.
As a hydrophilizing agent for a separation membrane for blood processing, a hydrophilic polymer having a small stimulus on blood is used. Examples of such a hydrophilic polymer include a polyvinylpyrrolidone, a polyvinyl alcohol and a polyethylene glycol.
Patent Literature 1 discloses that at least one of a polyalkylene oxide including polyethylene glycol, a polyvinylpyrrolidone, a polyvinyl alcohol, a poly(hydroxyethyl methacrylate), a polyacrylamide and a polyethylene imine is added for imparting hydrophilicity; and that incorporating such a hydrophilic polymer in the membrane hydrophilizes the surface of the membrane and provide an effect of suppressing adsorption of proteins to the membrane.
Patent Literatures 2 to 5 disclose, for further improving blood compatibility, methods for effectively suppressing adhesion of proteins and platelets by controlling balance between hydrophilicity and hydrophobicity by using a membrane having a vinylpyrrolidone unit and a hydrophobicity unit other than a polysulfone unit.
Even in such a separation membrane for blood processing prepared based on creative ideas as described above, initial performance may decrease or disappear with the passage of time. For example, a separation membrane is degraded by oxygen, temperature and light, etc. during transportation and storage. Storage for a long period may cause, even at about room temperature and with care, a chemical change more than a little in a separation membrane of a blood processing apparatus. In addition, during transportation, a separation membrane also has the potential to be exposed to high temperature.
If a separation membrane chemically changes, it is easily estimated that blood compatibility present in the beginning of storage reduces and a hydrophilic polymer is decomposed, with the result that the amount of eluted substances from the separation membrane increases. As a method for suppressing a chemical change of a separation membrane, Patent Literature 6 discloses that considering water content of the separation membrane, airtightness of a packaging material and the atmosphere within a package, a blood processing apparatus is kept under deoxidation conditions by packaging the apparatus together with a deoxidant, thereby reducing a change in amount of eluted substances from the separation membrane for a long time period.